1

NOTES ON BIOGAS TECHNOLOGY

Introduction

Properties of biogas

Feedstock for biogas: Aqueous wastes containing biodegradable organic matter,

animal & agro - residues

Microbial and biochemical aspects,

Operating parameters for biogas production.

Kinetics and mechanism

Dry and wet fermentation.

Digesters for rural application

High rate digesters for industrial waste water treatment.

TEXT BOOKS AND REFERENCES

1. Biotechnology Volume 8, H.J. Rehm and G. Reed, 1986, Chapter 5,

„Biomethanation Processes.‟ Pp 207-267

2. K. M. Mital, Non-conventional Energy Systems, (1997), A P H Wheeler

Publishing, N. Delhi.

3. K. M. Mital, Biogas Systems: Principles and Applications, (1996) New Age

International Publishers (p) Ltd, N. Delhi.

4. Nijaguna, B.T., Biogas Technology, New Age International publishers (P) Ltd.,

2002, Reprinted in 2009

References:

1. Effluent Treatment & Disposal: I Ch. E, U.K., Symposium Series No 96, 1986,

P 137-147, Application of anaerobic biotechnology to waste treatment and energy

production, Anderson & Saw.

2. „Anaerobic Rotating Biological Drum Contactor for the Treatment of Dairy Wastes‟,

S. Satyanarayana, K. Thackar, S. N. Kaul, S.D. Badrinath and N.G. Swarnkar, Indian

Chemical Engineer, vol 29, No 3, July-Sept, 1987

2

3. Energy Environment Monitor, 12(1), 45-51,„Biomethanation Technologies in

Industrial Water Pollution Control‟ A. Gangagni Rao, Pune.

4. „Biogas production from sugar mill sludge by anaerobic digestion and evaluation of

bio-kinetic coefficients‟, Tharamani. P, and Elangovan. R. Indian journal of

Environmental protection, 20, (10), 745-748, 2001.

5. „Biogas Production Technology: An Indian Perspective‟, B. Nagamani and K.

Ramasamy (TNAU), Current Science, Vol7, No1, pp 44-55 10th

July, 1999

6. Khandelwal K. C. and Mahdi, “Bio-gas Technology”, Tata McGraw-Hill publ. Co.

Ltd., New Delhi, 1986.

7. State-of-the-art of anaerobic digestion technology for industrial wastewater treatment -

KV Rajeshwari, M Balakrishnan, A Kansal, K Lata, … - Renewable and Sustainable

Energy Reviews, 2000 – Elsevier

8. Anaerobic digestion technologies for energy recovery from industrial wastewater - a

study in Indian context, Arun Kansal, K V Rajeshwari, Malini Balakrishnan, Kusum

Lata, V V N Kishore, TERI Information Monitor on Environmental Science 3(2): 67–75,

9. Biogas Purification and Bottling into CNG Cylinders: Producing Bio-CNG from

Biomass for Rural Automotive Applications, Virendra K. Vijay1,*, Ram Chandra1,

Parchuri M. V. Subbarao2 and Shyam S. Kapdi3 1Centre for Rural Development and

Technology, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi – 110 016,

The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE

2006)” C-003 (O) 21-23 November 2006, Bangkok, Thailand

3

10. Biogas scrubbing, compression and storage: perspective and prospectus in Indian

Context, S.S. Kapdi, V.K. Vijay*, S.K. Rajesh, Rajendra Prasad, Centre for Rural

Development and Technology, Indian Institute of Technology, New Delhi 110 016

Contents of „Biogas Technology‟ by B. T. Nijaguna, New Age International publishers

(P) Ltd., 2002, reprinted in 2009.

Introduction, Materials for Biomethanation and Products of Methanation, Kinetics and

Physico-Chemical Factors Affecting Biogasification, Bio-reactors, Design, Selection,

Construction and operation of Biogas Plants, Purification, Scrubbing, Compression and

Storage of Biogas, Utilisation Systems of Biogas, Ethanol.

K. M. Mital, Biogas Systems: Principles and Applications, (1996) New Age International

Publishers (p) Ltd, N. Delhi.

Contents:

1. An Overview of Biogas Technology

2. Microbiology of Anaerobic Digestion

3. Properties of Biogas and Methods For Its Purification

4. A Compendium of Biogas Plant Design

5. Design, Construction, Operation and Maintenance of Biogas Plants

6. Analysis of Factors Affecting Biogas Yield

7. Biogas Yield from Different Organic Wastes

8. Biogas Yield from Water Weeds

9. Biogas Generation from Industrial Wastes

10. Biogas Recovery from Sanitary Landfills

11. Applications and Usage Of Biogas

12. Potential of Biogas Plant Effluent As Enriched Fertilizer.

13. Approaches For Implementing Biogas Program Areas For Further Research And

Concluding Observations

4

INTRODUCTION: Production of a combustible gas by anaerobic digestion of aqueous organic matter

by mixed bacterial culture involving methane producers is called „biomethanation‟ and

the product is called „biogas‟

PROPERTIES OF BIOGAS: Composition: 60 to 70 per cent Methane, 30 to 40 per cent carbon dioxide, traces of

hydrogen sulfide, ammonia and water vapor.

It is about 20% lighter than air (density is about 1.2 gm/liter).

Ignition temperature is between 650 and 750 C.

Calorific value is 18.7 to 26 MJ/ m3 (500 to 700 Btu/ ft

3.)

Calorific value without CO2: is between 33.5 to35.3 MJ/ m3

Explosion limit: 5 to 14 % in air.

Removal of CO2: Scrubbing with limewater or ethanol amine solution.

Removal of H2S: Adsorption on a bed of iron sponge and wood shavings.

Air to Methane ratio for complete combustion is 10 to 1 by volume.

One cubic meter of biogas is equivalent to 1.613 liter of kerosene or 2.309 kg of LPG or

0.213 kW of electricity.

5

WHY Biomethanation in villages?

ENERGY RECOVERY, CLEAN BURNING

SUBSTITUTES FUELWOOD & DUNG CAKE AS

RURAL FUEL

HYGIENIC DISPOSAL OF ANIMAL WASTE

CONSERVATION OF MANURE VALUE

MILD CONDITIONS: 30o C, pH 6.8-7.2, FEED ONCE A

DAY

BURNER, MANTLE LAMP AVAILABLE; EASY GAS

PURIFICATION FEASIBLE

SUBSIDY IS AVAILABLE FOR RURAL FARM OR

FAMILY SIZE PLANT

DUAL FUEL ENGINE CAN PUMP WATER,

GENERATE POWER

BIOGAS TECHNOLOGY: SIMPLE & INDIGENOUS

DUNG

WATER

BIOGAS

PLANT BIOGAS

PURIFY I. C. ENGINE +

PUMP OR

I. C. ENGINE +

GENERATOR

COOKING

LIGHTING

FUEL FOR

KILN

FURNACE

ETC.

TO

COMPOST PIT

(MANURE)

6

WET ORGANIC WASTE AS FEED FOR BIOGAS PLANT

ANIMAL WASTES: Excreta of cow, pig, chicken etc

MANURE, SLUDGE: Canteen and food processing waste, sewage

MUNICIPAL SOLID WASTE: After separation of non-degradable

WASTE STARCH & SUGAR SOLUTIONS: Fruit processing, brewery, press mud

from sugar factory etc

OTHER INDUSTRIAL EFFLUENTS (B O D): pulp factory waste liquor,

leather industry waste, coal washery wastewater etc

Commonly Used Feed for Biomethanation:

Animal Wastes,

Crop Residues,

Urban Wastes,

Food and Agro - Industry Wastes.

(Mital, Ch. 7 To 10)

7

MICROBIOLOGIAL ASPECTS OF BIOMETHANATION

The biomethanation of organic matter in water is carried out in absence of dissolved

oxygen and oxygenated compounds like nitrate and sulphate. The mixed groups of

bacteria are naturally occurring in the cow dung slurry and decomposition in three stages

finally produces a gas mixture of methane and carbon dioxide. Initially larger molecules

are hydrolysed to simpler molecules which in turn are decomposed to volatile fatty acids

like acetic acid, propionic acid etc. by a second set of bacteria. Methane forming bacteria

can convert acetic acid, hydrogen and carbon dioxide and produce methane.

Figure 1: The process of methanogenesis (After GTZ, 1999)

HYDRLYSIS OF BIOPOLYMERS TO MONOMERS

CONVERSION OF SUGARS, AMINO ACIDS, FATTY ACIDS TO HYDROGEN,

CO2, AMMONIA AND ACETIC, PROPIONIC AND BUTIRIC ACIDS

CONVERSION OF H2, CO2, ACETIC ACID TO CH4 AND CO2 MIXTURE

8

Methanogenesis is a microbial process, involving many complex, and differently

interacting species, but most notably, the methane-producing bacteria. The biogas process

is shown below in figure 1, and consists of three stages; hydrolysis, acidification and

methane formation.

In the first stage of enzymatic hydrolysis, the extracellular enzymes of microbes, such as

cellulase, protease, amylase and lipase externally enzymolize organic material. Bacteria

decompose the complex carbohydrates, lipids and proteins in cellulosic biomass into

more simple compounds. During the second stage, acid-producing bacteria convert the

simplified compounds into acetic acid (CH3COOH), hydrogen (H2), and carbon dioxide

(CO2). In the process of acidification, the facultatively anaerobic bacteria utilize oxygen

and carbon, thereby creating the necessary anaerobic conditions necessary for

methanogenesis. In the final stage, the obligatory anaerobes that are involved in methane

formation decompose compounds with a low molecular weight, (CH3COOH, H2, CO2), to

form methane (CH4) and CO2 (Gate, 1999).

The resulting biogas, sometimes referred to as 'gobar' gas, consists of methane and carbon

dioxide, and perhaps some traces of other gases, notably hydrogen sulphide (H2S). Its

exact composition will vary, according to the substrate used in the methanogenesis

process, but as an approximate guide, when cattle dung is a major constituent of

fermentation, the resulting gas will be between 55-66% CH4, 40-45% CO2, plus a

negligible amount of H2S and H2 (KVIC, 1993). Biogas has the advantage of a potential

thermal efficiency, given proper equipment and aeration, of 60%, compared to wood and

dung that have a very low thermal efficiency of 17% and 11% respectively (KVIC,

1993).

Methanogenesis or more particularly, the bacteria involved in the fermentation process

are sensitive to a range of variables that ultimately determine gas production, and it is

worth briefly outlining these factors. Temperature is perhaps the most critical

consideration. Gasification is found to be maximized at about 35oC, and below this

temperature, the digestion process is slowed, until little gas is produced at 15oC and

under. Therefore in areas of temperature changes, such as mountainous regions, or winter

conditions that may be more accentuated inland, mitigating factors need to be taken into

9

account, such as increased insulation (Kalia, 1988), or the addition of solar heaters to

maintain temperatures (Lichtman, 1983).

Loading rate and retention period of material are also important considerations. In the

KVIC model, retention ranges between 30-55 days, depending upon climatic conditions,

and will decrease if loaded with more than its rated capacity (which may result in

imperfectly digested slurry). KVIC state that maximum gas production occurs during the

first four weeks, before tapering off, therefore a plant should be designed for a retention

that exploits this feature. Retention period is found to reduce if temperatures are raised, or

more nutrients are added to the digester. Human excreta, due to its high nutrient content,

needs no more than 30 days retention in biogas plants (KVIC, 1983).

Various factors such as biogas potential of feedstock, design of digester, inoculum, nature

of substrate, pH, temperature, loading rate, hydraulic retention time (HRT), C : N ratio,

volatile fatty acids (VFA), etc. influence the biogas production.

Meher et al. reported that the performance of floating dome biogas plant was better than

the fixed dome biogas plant, showing an increase in biogas production by 11.3 per cent,

which was statistically significant. Furthermore, the observed reduction in biogas yield

was due to the loss of gas from the slurry-balancing chambers of fixed dome plant.

Dhevagi et al. used different feedstocks like cow dung, buffalo dung, dry animal waste,

stray cattle dung, goat waste, and poultry droppings for their biomethanation potential

and observed that poultry droppings showed higher gas production. Earlier Yeole and

Ranade compared the rates of biogas yield from pig dung-fed and cattle dung-fed

digesters and reported that the biogas yield was higher in the former. They attributed this

higher biogas yield to the presence of native microflora in the dung. Shivraj and

Seenayya reported that digesters fed with 8 per cent TS of poultry waste gave better

biogas yield, and attributed the lower yield of biogas at higher TS levels to high ammonia

content of the slurry

10

BIOLOGICAL MODELING

11

12

13

The modeling and its simulation referred to is from the following paper:

14

Operating parameters affecting the biogas production:

1. Temperature is an important parameter. Mesophilic methane producing bacteria

grow at an optimum temperature of 35oC the gas production rate drops very much

when temperture is less than 10oC.

2. pH range of the waste water should be in the range of 6.8 to 7.8 as excess acid

state hampers the methane producing bacteria and the balance of nutrients is

disturbed.

3. Ratio of carbon to nitrogen in the waste water influent or C/N ratio is 30:1 and if

nitrogen content in ammoniacal form is less the bacterial growth is affected and

the process slows down.

4. Proportion of solids to water: This is found to be not more than 10 per cent for

optimum operation of digester to ensure sufficient decomposition of „volatile

solids‟ and rate of production of gas.

5. Retention time: The ratio of volume of slurry in the digester to the volume fed

into and removed from it per day is called retention time. Thus a 20 liter digester

is fed at 4 liters per day so that the volume of digester is constant the retention

time is 5 days. The required retention time is normally 30 days for mesophilic

(25-35oC) conditions.

6. Volumetric organic loading rate: This can be expressed as kg Vs per volume per

day based on the % weight of organic matter added each day to the digester

volume.

Digester loading rate %= (Per cent of organic matter in feed)/(Retention Time)

Loading rate range is 0.7 to25 kg VS/ m3

/ Day

15

Kinetics of anaerobic fermentation

Several kinetic models have been developed to describe the anaerobic fermentation

process. Monod101

showed a hyperbolic relationship between the exponential microbial

growth rate and substrate concentration. In this model, the two kinetic parameters,

namely, microorganisms growth rate and half velocity constant are deterministic in

nature, and these predict the conditions of timing of maximum biological activity and its

cessation. This model can be used to determine the rate of substrate utilization (rS) by the

equation:

rS = qmax ´ Sx/K + S,

where S is limiting substrate concentration, K is half constant, x is concentration of

bacterial cells, and qmax is maximum substrate utilization rate.

The above equation is applicable for low substrate concentration.

However for high substrate concentration, the equation is re-written as:

rS = qmax · x.

The Monod model suffers from the drawback that one set of kinetic parameters are not

sufficient to describe biological process both for short- and long-retention times, and that

kinetic parameters cannot be obtained for some complex substrates. To alleviate

limitations of the Monod model while retaining its advantages, Hashimoto102

developed

an alternative equation, which attempts to describe kinetics of methane fermentation in

terms of several parameters. According to this equation, given below, for a given loading

rate So/q daily volume of methane per volume of digester depended on the

biodegradability of the material (Bo) and kinetic parameters µm and K.

rV = (Bo ´ So/q ) · {1– (K/q µm – 1 + K)}

where,

rv is volumetric methane production rate, l CH4 l– 1

digester d– 1

So is influent total volatile solids (VS) concentration,

g l– 1

Bo is ultimate methane yield, l CH4 g– 1

VS added as q

16

q is hydraulic retention time d– 1

µm is maximum specific growth of microorganism d– 1

K is kinetic parameter, dimensionless.

****** KINETICS OF ANAEROBIC FERMENTATION (Reference: Mital, pp 36-39):

Rate of substrate Utilization,

rs = Qmax * (Sx) / (K+S) ---(1)

Where S is limiting substrate concentration

K is half life constant

X is concentration of bacterial cells

Qmax is maximum substrate utilization rate

For low substrate concentration, this equation is valid. For high substrate concentration, it

becomes as follows:

rs = Qmax*x ----(2)

The above model known as Monod model has limitations. For complex substrates, kinetic

parameters cannot be obtained for the entire concentration range.

Chen and Hashimoto, Biotechnology Bio-engineering Symposium 8, (1978) p 269-

282 and Biotechnology Bioengineering (1982) 24: 9-23

Volumetric methane rate in cubic meter gas per cubic meter of digester volume

V = (Bo So / HRT)[1- K / (HRT*m-1+K)]

Bo = Ultimate methane yield in cubic meters methane (Varies from 0.2 to 0.5)

So = Influent volatile solids concentration in kgVS/m3

(Loading rate range = 0.7 to 25 kg VS/m3 d)

HRT = Hydraulic retention time in days

K = Dimensionless kinetic parameter, for cattle dung, K= 0.8+ 0.0016e0.06 So

17

m = Maximum specific growth rate of the microorganism in day-1

Different types of biogas plant recognized by MNES (Ministry of Non-

Conventional Energy Sources). After Gate, 1999.

1. Floating-drum plant with a cylinder digester (KVIC model).

2. Fixed-dome plant with a brick reinforced, moulded dome (Janata model).

3. Floating-drum plant with a hemisphere digester (Pragati model).

4. Fixed-dome plant with a hemisphere digester (Deenbandhu model).

5. Floating-drum plant made of angular steel and plastic foil (Ganesh model).

6. Floating-drum plant made of pre-fabricated reinforced concrete compound

units.

7. Floating-drum plant made of fibreglass reinforced polyester.

18

19

RURAL DIGESTERS ACCEPTED BY MNES:

(Digesters for rural application)

1 KVIC (FLOATING DOME)

MASONRY CYLINDRICAL TANK

ON ONE SIDE INLET FOR SLURRY

OTHER SIDE OUTLET FOR SPENT SLURRY

GAS COLLECTS IN INVERTED ‘DRUM’ GAS HOLDER OVER

SLURRY

GAS HOLDER MOVES UP & DOWN DEPENDING ON

ACCUMULATION OF GAS /DISCHARGE OF GAS, GUIDED BY

CENTRAL GUIDE PIPE

GAS HOLDER (MILD STEEL): PAINTED ONCE A YEAR.

K V I C Mumbai

MEDIUM FAMILY SIZE BIOGAS PLANT HAVING GAS DELIVERY OF

3 M3 /DAY REQUIRES 12 HEAD OF CATTLE AND CAN SERVE A

FAMILY OF 12 PERSONS

TECHNICAL DETAILS OF A 3 M

3 /DAY BIOGAS PLANT OF

FLOATING DRUM DESIGN

Name of the model KVIC Model

Size for 3m3 / day gas delivery 4.15m high, 1.6m dia, Volume 8.34m

3

Inlet pipe 0.1m dia, 4m long

Inlet tank 0.75m dia, 1m high

Outlet pipe 0.1m dia, 1.1 m long

Retention period 30 to 50 days

Gas Holder 1.5 m dia, 1m high

Construction of gas holder MS sheet & angles, fabricated.

Constr. & layout, digester Brick, cement, digester below G. level

20

2. JANATHA (FIXED DOME)

DIGESTER WELL BELOW GROUND LEVEL

FIXED DOME GAS HOLDER BUILT WITH BRICK & CEMENT

BIOGAS FORMED RISES PUSHES SLURRY DOWN

DISPLACED SLURRY LEVEL PROVIDES PRESSURE-UPTO THE POINT

OF ITS DISCHARGE/ USE

3 DEENABANDU (FIXED DOME, MINIMISES SURFACE AREA)

FIXED DOME PLANT, MINIMISES SURFACE AREA BY JOINING THE

SEGMENTS OF TWO SPHERES OF DIFFERENT DIAMETERS AT THEIR

BASES

FIXED MASONRY DOME REQUIRES SKILLED WORKMANSHIP AND

QUALITYMATERIALS TO ELIMINATE CHANCE OF LEAKAGE OF GAS

AFPRO, 25/1A, Institutional Area, D block, Panka Rd, Janakpury, N.Delhi.

BIOGAS inlet

outlet

21

4 PRAGATI

COMBINES FEATURES OF KVIC & DEENABANDU, MAHARASHSTRA

LOWER PART: SEMI-SPHERICAL IN SHAPE WITH A CONICAL BOTTOM

UPPER PART: FLOATING GAS HOLDER

POPULARISED IN MAHARASHTRA, UNDARP, PUNE

5 FERROCEMENT DIGESTER:

CAST SECTIONS, MADE FROM A REINFORCED (MORTAR+WIRE

MESH)- COATED WITH WATER PROOFING TAR

S E R I, ROORKEE

6 FRP DIGESTER:

FIBER REINFORCED PLASTIC MADE BY CONTACT MOULDING

PROCESS

7 UTKAL / KONARK DIGESTER

Reference: „Konark biogas plant-A user friendly model‟ Mohanty, P.K., and Choudury,

A. K, (Orissa Energy Dev. Agency), Journal of Environmental Policy and Studies 2(1);

15-21

Konark Biogas plant:

SPHERICAL IN SHAPE WITH GAS STORAGE CAPACITY OF 50%

CONSTRUCTION COST IS REDUCED AS IT MINIMIZES SURFACE AREA

BRICK MASONRY OR FERROCEMENT TECHNOLOGY

A PERFORATED BAFFLE WALL AT THE INLET PREVENTS SHORT

CIRCUITING PATH OF SLURRY (OPTIONAL)

22

8 FLEXIBLE PORTABLE NEOPRENE RUBBER MODEL:

FOR HILLY AREAS, MINIMIZES TRANSPORT COST OF MATERIALS

BALLOON TYPE, INSTALLED ABOVE GL, MADE OF NEOPRENE

RUBBER

FOR FLOOD PRONE AREAS, UNDERGROUND MODELS NOT SUITABLE

SWASTHIK COMPANY OF PUNE DESIGN

23

24

25

HIGH RATE DIGESTERS FOR WASTE WATER TREATMENT:

1. ANAEROBIC FILTER (UPFLOW and DOWNFLOW)

2. UPFLOW ANAEROBIC SLUDGE BLANKET DIGESTER( UASB)

3. ANAEROBIC LIQUID FLUIDISED/ EXPANDED BED DIGESTER

4. ANAEROBIC ROTATING DISC CONTACTING DIGESTER

5. ANAEROBIC MEMBRANE DIGESTER

6. ANAEROBIC CONTACT DIGESTER

Effluent Treatment & Disposal: I Ch. E, U.K., Symposium Series No96,

1986., P 137-147, Application of anaerobic biotechnology to waste

treatment and energy production Anderson & Saw.

Energy & Environment Monitor, 12(1) 45- 51, „Biomethanation

Technologies in industrial water pollution Control‟ A.Gangagni Rao, Pune.

Techniques for enhancing biogas production

Different methods used to enhance biogas production can be classified into

the following categories:

(i) Use of additives.

(ii) Recycling of slurry and slurry filtrate.

(iii) Variation in operational parameters like temperature, Hydraulic

retention time (HRT) and particle size of the substrate.

(iv) Use of fixed film / biofilters.

26

HIGH RATE DIGESTERS FOR WASTE WATER TREATMENT:

1 ANAEROBIC FILTER (UPFLOW and DOWNFLOW)

ANAEROBIC FILTER CONTAINS A SOLID SUPPORT OR PACKING

MATERIAL IT WAS DEVELOPED BY YOUNG & MC CARTHY IN 1967

WASTEWATER FLOWS FROM BOTTOM UPWARDS THROUGH THE

PACKING, GAS SEPARATES, BACTERIA ARE RETAINED MOSTLY

IN SUSPENDED FORM,HRT RANGE OF 0.5 TO12 DAYS IS OBTAINED

SINCE SUSPENDED GROWTH TENDS TO COLLECT NEAR THE

BOTTOM OF THE REACTOR, ACTIVITY IS HIGHER THERE.

TYPICAL ORGANIC LOADING RATE OF 1 TO 40 KG COD/

M3/DAYAND A SRT OF 20 DAYS IS ACHIEVED.

AVOIDANCE OF PLUGGING DUE TO ACCUMULATION OF SOLIDS

IN THE PACKING MATERIAL AND ENSURING AN ADEQUATE

FLOW DISTRIBUTION IN THE BOTTOM OF THE REACTOR ARE

THE LIMITATIONS OF THIS.

27

2 UPFLOW ANAEROBIC SLUDGE BLANKET DIGESTER (UASB)

UASB REACTOR IS BASED ON SUPERIOR SETTLING PROPERTIES

OF THE SLUDE

INFLUENT FED INTO THE REACTOR FROM BELOW LEAVES AT

THE TOP VIA AN INTERNAL BAFFLE SYSTEM FOR SEPARATION

OF THE GAS, SLUDGE AND THE LIQUID

GAS SEPARATED FROM SLUDGE, COLLECTED BENEATH PLATES

IN QUIET SETTLING ZONE, SLUDGE SEPARATES, SETTLES BACK

TOWRDS DIGESTION ZONE.

ORGANIC LOADING RATES OF 10 TO 30 KG COD /M3 DAY

REACTOR MIXING SHOULD BE ONLY BY THE GAS PRODUCTION

HRTRANGE OF 0.5 TO 7 DAYSS IS FEASIBLE WITH EXCEL.

SETTLING SLUDGE AND A SRT OF 20 DAYS(AT 35 0

C)

REF: TIDE, VOL9, NO4, DEC.1999, PAGE 232

28

3 ANAEROBIC LIQUID FLUIDIZED/ EXPANDED BED DIGESTER

ACTIVE BIOMASS IS ATTACHED TO SURFACE OF SAND

PARTICLES THAT ARE KEPT IN SUSPENSION BY UPWARD

VELOCITY OF LIQUID FLOW

DEGREE OF BED EXPANSION IN EXPANDED BED IS 10-20% AND IN

FLUIDIZED BED IT IS 30-100%

BIOMASS RETENTION IN THE REACTOR IS EFFICIENT ,SRT OF 30

DAYS

PARTICLES PROVIDE LARGE SURFACE AREA FOR MICROBIAL

GROWTH AND BETTER MIXING COMPARED TO PACKED BED, HRT

RANGE OF 0.2 TO 5.0 DAY ACIEIVED.

TYPICAL RANGE OF LOADING RATE OF 1 TO 100 KG COD/M3 /DAY

REF: COMPREHENSIVE BIOTECHNOLOGY-MURRAY MOO YOUNG,

VOL 4, PAGES 1017-1027.

29

4. ANAEROBIC ROTATING BIOLOGICAL DISC CONTACTOR

ANAEROBIC ROTATING BIOLOGICAL CONTACTOR CONSISTS OF A

SERIES OF DISCS OR MEDIA BLOCKS MOUNTED ON A SHAFT WHICH IS

DRIVEN SO THAT THE MEDIA ROTATES AT RIGHT ANGLES TO THE

FLOW OF SEWAGE. THE DISCS OR MEDIA BLOCKS ARE NORMALLY

MADE OF PLASTIC (POLYTHENE, PVC, EXPANDED POLYSTYRENE) AND

ARE CONTAINED IN A TROUGH OR TANK SO THAT ABOUT 40% OF

THEIR AREA IS IMMERSED.

Reference Article:

„Anaerobic Rotating Biological Drum Contactor for the Treatment of Dairy

Wastes‟, S. Satyanarayana, K. Thackar, S.N.Kaul, S.D.Badrinath and N.G.

Swarnkar, (NEERI) Indian Chemical Engineer, vol 29, No 3, July-Sept,

1987.

30

5. ANAEROBIC MEMBRANE DIGESTER

SUSPENDED GROWTH REACTOR, COMBINED WITH A SEPARATOR

EXTERNAL ULTRA / MICROFILTRATION MEMBRANE UNIT FOR

SOLID-LIQUID SEPARATION

PERMEATE BECOMES THE EFFLUENT AND THE BIOMASS IS

RETURNED TO THE REACTOR

MEMBRANE UNIT ROVIDES POSITIVE BIOMASS RETENTION AND

PARTICULATE FREE EFFLUENT

31

6 ANAEROBIC CONTACT DIGESTER

BIOMASS SETTLED IN A SECOND TANK, RECYCLED TO THE

DIGESTER.

RECYCLE GIVES HIGHER SRT AND EFFICIENCY

MIXING IN THE FIRST TANK AND EFFICIENCY OF SETTLING IN

THE SECOND TANK IMPROVES PERFORMANCE.

REQUIRE HRT OF 10 DAYS OR MORE.

32

33

34

Comments on Rural biogas plants:

Biogas has shown to be a useful component in the rural economy in India,

though its application is logistically difficult. Ill-co-ordinated dissemination

has led to high rates of non-functioning plants, and may endanger further

uptake, as such, its status as a fuel remains marginal.

Participation in biogas technology varies across socio-economic groups, and

across regions. Despite a well-intentioned attempt to cater for the energy

needs of rural India, and particularly the poor, as defined by 'scheduled caste'

and 'scheduled tribe', the biogas programme has not appeared to meet these

needs on any meaningful scale, through insurmountable constraints

associated with their very marginality, paradoxically. Limited success has

occurred in other agricultural groups.

Further, the essential 'commodification' of dung, which has occurred since

the introduction of biogas systems may impact detrimentally upon the

poorest families, who may experience a scarcity of the fuel once gathered for

free. The need to provide rural India with a viable and sustainable source of

fuel has perhaps never been more urgent, yet curiously, this is not reflected

in current literature, as biogas seemingly drops out of journals in the 1990's,

as a subject to be written about. Therefore, the very current situation

regarding the status of biogas technology in India is unknown, though

dissemination is still being undertaken. Bapu's (Gandhi's) dream therefore

remains largely unrealized, though 'small steps' may have been achieved.

35

1. What properties of biogas have to be improved before it is used

as an engine fuel?

2. Write short notes on (i) Feedstock for biogas, (ii) Dry and wet

fermentation, (iii) Microbial and biochemical aspects.

3. Discuss the operating parameters for biogas production by

anaerobic digestion.

4. What criteria are applied in selecting a rural biogas plant of a

small family size?

5. Why biogas is not supplied in cylinders like LPG? Can we use

same stove for both?

6. Explain hydraulic and solid retention time for a fixed film biogas

digester.

7. In a flood prone area, what type of small biogas plant would

you use?

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